This invention pertains to current limiting fuses, and more particularly to such fuses having filler materials containing water that absorb energy during fuse operation by having their chemically bonded water molecules driven off as steam.
Fuses of the kind described can be used to protect various electrical devices so as to limit the amount of current flowing through those devices when a fault occurs. By limiting such overcurrents, the fuses also limit the amount of energy flowing in the electrical circuit in which they are installed. During fuse operation, energy of the electric circuit is absorbed by the fuse to melt one or more portions of fusible elements contained within the fuse. After a first portion of a fusible element is melted, the voltage across the melted portion rises very rapidly, and an arc is formed across the melted portion, subsequently vaporizing both melted and unmelted portions of the fusible element. During this time, a great quantity of heat is generated by the electrical arc. To operate successfully, the current limiting fuse must absorb appreciable amounts of the heat energy generated by the electric arc, to preclude internal carbonization of the fuse and also to preclude the fuse from burning. Filler materials of the type containing water are heated upon contact with an electric arc, converting their contained water to steam, thereby absorbing significant amounts of heat energy. It is crucial then that a fuse filler material of the above-described type retain its moisture content until an arc is initiated. If the water content is not available to absorb heat energy through the above-described phase change, then current continues to flow through the molten metal of the fuse link, and with the attendant rise in arc voltage as described above, the arc continues to heat the melted fusible material until it vaporizes. After the metal has been vaporized, it can still support a flow of current, with the arc occurring in the filler material of the fuse where the molten metal existed previously. The fuse will then continue to absorb energy, and the arc will continue to burn and lengthen, until the path becomes too long for the resulting voltage to sustain the arc. Arcs, inadequately quenched within a fuse have been observed to travel outside the fuse casing, melting the fuse terminals, thereby causing destruction to surrounding equipment.
It has been observed that prior art fuse filler materials of the above-described type lose a substantial portion of their contained water during prolonged low level overcurrent fuse operation. During this time, the fuse has been operated above its current rating, and is heated above its rated temperature. Depending on the design of the fusible element and the amount of overcurrent conducted through the fuse, the fuse filler could be heated for several minutes before the fusible element severs, and an arc is formed within the fuse. During this time, water contained within the fuse filler is driven off as steam, and hence is not available at the time an arc is formed within the fuse.
Further problems are encountered with fuses supplied to the mining industry. Under the Federal Coal Mine Health and Safety Act of 1969, fuses used in mines must be approved by the Bureau of Mines, according to the testing requirements for approval, as reported on page 7564 of the Federal Register, Volume 37, No. 74 dated Saturday, Apr. 15, 1972. In one test, the fuses must be preconditioned (prior to testing) by heating to 90.degree. centigrade for 24 hours. The fuses must then be tested within one hour after removal from the preconditioning chamber.
It has been found that prior art fuse filler materials lose significant portions of their water content during the bake-out or preconditioning process required for testing. One filler material, widely used in the fuse industry, calcium sulfate (Ca S0.sub.4) loses 67% of its water of hydration after being heated to 90.degree. centigrade for twenty-four hours. Thus, the arc quenching ability of calcium sulfate or its variations commonly known as plaster of paris or gypsum, while generally performing satisfactorily under lower temperature conditions, is significantly degraded when prepared in accordance with the aforementioned testing requirements of the Bureau of Mines. An improved filler material of this type would contain a greater number of water molecules at the time arcing is initiated in the fuse, even after a pretreatment of the above-described type. Also, further energy absorption is possible if the melting temperature of the filler material is low enough to be exceeded during arc formation, such that the filler melts, absorbing from the arc, an amount of energy equal to the material's energy of reaction.
Another problem is encountered in providing fuses to the mining industry and other users who require fuse protection for direct current circuits. Popular filler materials of the type containing water, while satisfactorily interrupting alternating currents do not appear to absorb amounts of direct current arc energy necessary to avoid internal carbonization or burning during fuse operation, particularly fuse operation responding to prolonged low-level overcurrents.
Further, it is desirable to have a single type of fuse which is capable of operating successfully in both direct current and alternating current circuits.